Colton Harvey was charged in January for the shooting death of his 16-year-old sister Candace, shown here.

RUSSELLVILLE, Ark. — An Arkansas teen pleaded guilty Wednesday to murdering his sleeping sister and was sentenced to 45 years in prison for a crime he still hasn't fully explained.

Colton Harvey, 15, grabbed his father's .22-caliber rifle one January morning while his parents were out grocery shopping. He walked into his 16-year-old sister Candace's room, pointed it at her forehead and fired. She awoke with a scream, so he shot her in the head twice more.

He threw some clothes and ammunition in his father's pickup truck and took off, driving first into the hills but then to the sheriff's office, where he chickened out in the parking lot. He drove to a friend's for some chewing tobacco – a vice that led to his parents grounding him days earlier – and then back to the sheriff's, where this time he found the courage to go in and confess.

"I don't know why I did it. It just happened," Harvey told state police investigator Corey Mendenhall hours later, according to a transcript of the interview in which he described in detail what happened that morning. The Associated Press obtained the transcript under a Freedom of Information Act request.

Harvey told Mendenhall he deserved the same fate.

"I should get done to me what I did to her," Harvey said.

Prosecutors initially charged the teen with first-degree murder, which carries a maximum penalty of life without parole. But they later worked out a plea bargain with his attorney, and on Wednesday, a judge sentenced Harvey to 30 years for second-degree murder plus 15 more because he used a gun.

The lanky, blond Harvey teared up as he addressed Judge William Pearson, at one point raising his handcuffed hands to his face so he could dab his eyes with a tissue.

"You stated that you murdered your sister. Is that correct?" Pearson asked.

Harvey paused, then almost whispered, "Yes, sir."

"How far did you get in school?" Pearson asked Harvey, who responded so quietly that the judge had to repeat some of his answers.

As Harvey replied, "ninth grade," his mother sobbed.

Harvey told the state police investigator that his parents grounded him a few days before the shooting when they found out he was using smokeless tobacco. He stewed in his room, staring at the wall.

The morning he killed Candace, his parents woke him up to tend to jerky from a deer he had killed the weekend before.

"You've got to be angry to be able to shoot a gun at somebody," Mendenhall, the state police investigator, told him a few hours after he killed Candace. "I mean, you're used to shooting deer and stuff and I know you're, you're not angry at the deer. But we're talking about your sister here. Do you love your sister?"

"Yeah," he said.

Investigators found her body in a bedroom at the family's home near Ozark, a town of about 3,600 roughly 120 miles northwest of Little Rock.

More details about the shooting came loose after Wednesday's hearing, when the judge also unsealed court documents that he previously ordered be kept out of the public's eye.

And yet, the question of why Harvey shot his sister remained unanswered.

"He never did give what I would consider to be a clear motive," the prosecutor, David Gibbons, said after Wednesday's hearing.

Harvey's attorney, Bill James, said there is a history of mental illness in Harvey's family, but he said an expert wasn't able to give his client a diagnosis because of his young age.

"Every time I've ever seen him, he's cried," James said. "And it's not, `Woe is me.' It's about what he's done to his mom and what he's done to his family."

A state review of Harvey's mental health noted that he was depressed after being jailed and that he said he had lost consciousness playing football in junior high school. But it found nothing on which to blame the shooting.

His defense attorney said Harvey never had any run-ins with the law before the shooting.

"I think his biggest problem was talking in class prior to this," James said.

His parents had only recently discovered he was using smokeless tobacco.

"I don't see why they won't let me do it. I've done it since third grade," Harvey told the state police investigator.

Harvey will head to a county jail until he's transferred to the Division of Youth Services, where he'll remain at least until he turns 16, James said. He can head to a state prison after that.

Harvey's mother cried throughout Wednesday's proceedings that took away her son after she lost her daughter.

"The situation doesn't lend itself for anybody to be happy," Gibbons, the prosecutor, said. "If there was somebody happy, absolutely happy, then an injustice would have been done."